Field of the Invention
The present invention concerns an injection molding machine for a thermoplastic resin.
In thermoplastic resin molding, an injection method has been known as a typical process for molding complicated articles of three dimensional shapes. As is well-known, in an injection molding method, a thermoplastic resin material is melted, injected at a high injection pressure ln a clamped mold and then cooled to solidify in the mold to obtain molded articles with stable dimensional accuracy and having fine appearances with no sink marks, burs and the like.
To this end, a clamping force capable of withstanding a high injection pressure is required, and a strong mold is also demanded.
Usually, irrespective of the scale of machines, an injection unit having an injection pressure of from 1200 to 2500 kg/cm2 is provided. In this case, in order to increase the injection pressure in the same machine, the screw diameter has to be reduced. Since an injection pressure and an injection capacity are in a reverse proportion to each other, a high injection pressure is required for obtaining articles of stable dimensional accuracy, and only small-sized articles can be molded at present, even when strong and relatively large molding machines are used.
The reason why such a high injection pressure is required in existing molding machines is that it is intended to utilize the resin pressure for the operation of charging a molten resin into a mold cavity serving as a heat exchanger and for the prevention of sink marks on the surfaces of molded articles.
On the other hand, since the thermoplastic molten resin is deprived of heat by the mold during injection, it has a tendency to form skin layers on both upper and rear surfaces thereof which increase its viscosity in proportion to the flowing length thereof. Accordingly, the skin layers on both upper and rear surfaces harden at the time of completion of charging of the resin into the mold cavity and the injection pressure transmitting layer is narrowed.
Therefore, a high injection pressure is further required for expanding the resin from the central layer to the outer surface in order to resist the nature of the resin of cooling.fwdarw.solidification.fwdarw.shrinkage, and avoid sink marks on the surface caused by the shrinkage from the inside of the wall thickness of the molding articles. Accordingly, only small-sized articles can be obtained as compared with clamping pressure (ton) and injection pressure (ton) of the molding machine.
As described above, since the flowing length is limited in proportion to the thickness of the molded products, pinpoint gates are required at a number of positions in large-sized molding articles even when a large scale molding machine is used.
Further, it is desired for most large-sized molding articles to have less weight and increased strength, and to not thermally expand when heated. As a countermeasure, glass fibers (1 mm-7 mm in length) have recently been used as reinforcing composite materials in the field concerned. However, in a pinpoint gate mold for molding a molten resin mixed with glass fibers to make a composite material, the gate is enlarged, since gate abrasion takes place due to the flow of the glass fibers during injection. Therefore, when the molding articles are released from the mold (mold opening operation), no satisfactory gate-tearing can be attained to cause peeling or cracking at the gate portion, often resulting in failure to obtain good molded articles.
For such reasons, a reinforcing composite material such as glass fibers that promote abrasion of molds cannot be used for molding large-sized-articles, requiring strength at reduced thickness and weight, by the use of pinpoint gates. That is, molded resin articles having a large area, reduced thickness and weight, and a large strength can not presently be obtained by injection molding.